1. Field of the Invention
The present invention relates to magnetometers and, more particularly, to a semiconductor giant magneto-impedance (GMI) magnetometer.
2. Description of the Related Art
A magnetometer is a device that measures the strength of an external magnetic field. There are a number of different approaches to measuring magnetic fields, and various different types of magnetometers have been developed based on these different approaches. One type of magnetometer is based on the giant magneto-impedance (GMI) effect.
The GMI effect results when a high-frequency alternating current passes through a soft ferromagnetic material. In the absence of an external magnetic field, the soft ferromagnetic material has a baseline impedance. However, in the presence of an external magnetic field, the impedance of the soft ferromagnetic material changes significantly. This change in the impedance in response to an external magnetic field is known as the GMI effect.
FIG. 1 shows a diagram that illustrates an example of a prior art GMI magnetometer 100. As shown in FIG. 1, GMI magnetometer 100 includes a soft ferromagnetic structure 110, and an alternating current source 112 that is connected to soft ferromagnetic structure 110. GMI magnetometer 100 also includes a sense coil 114 that is wrapped around soft ferromagnetic structure 110, and a processing circuit 116 that is connected to sense coil 114.
In operation, alternating current source 112 drives a high-frequency alternating current through soft ferromagnetic structure 110, which generates an alternating voltage across soft ferromagnetic structure 110. In the absence of an external magnetic field, soft ferromagnetic structure 110 has a base line impedance. In the presence of an external magnetic field, the impedance of soft ferromagnetic structure 110 changes. Materials which have a higher circumferential or transverse permeability provide a greater change in the impedance.
The alternating current passing through soft ferromagnetic structure 110 generates an alternating magnetic field, which changes in response to changes in the impedance of soft ferromagnetic structure 110. The alternating magnetic field induces an alternating current and an alternating voltage in sense coil 114 that tracks the alternating current passing through soft ferromagnetic structure 110 and the alternating voltage across soft ferromagnetic structure 110.
Processing circuit 116 detects phase changes between the alternating current and alternating voltage in sense coil 114, and determines changes in impedance from the changes in phase. Processing circuit 116 then determines changes in the magnitude of an external magnetic field from the changes in impedance, and generates an output voltage that is proportional to the magnitude of the external magnetic field.
Alternatively, in an H-field detection scheme, sense coil 114 is omitted, and processing circuit 116 is electrically connected to the opposite ends of ferromagnetic structure 110. In this case, processing circuit 116 measures the induced change in ac impedance directly across ferromagnetic structure 110.
For example, Aichi Steel manufactures products (e.g., compass 360 701 and motion sensor A603 0601) that utilize a GMI magnetometer where soft ferromagnetic structure 110 is implemented with an amorphous wire of FeCoSiB, which is cold drawn to a radius of approximately 15-30 μm and then tension annealed to obtain precise circumferential anisotropy.
Sense coil 114, in turn, is implemented with a wire that is insulated from and wrapped around the amorphous wire to form a GMI sensor. The GMI sensor is attached to a printed circuit board, with the amorphous wire of the GMI sensor being connected to an alternating current source, and the wrapped wire that forms sense coil 114 of the GMI sensor being electrically connected by way of printed circuit board traces to integrated circuits that include processing circuit 116.
Although GMI magnetometers measure the strength of an external magnetic field, current-generation GMI magnetometers tend to be bulky and expensive to manufacture. Thus, there is a need for a smaller and less expensive GMI magnetometer.